I think they did that just so you can get the achievement where you kill the ruler the turn they show up. I don't even think that's possible without that.
Nothing really prevents you from leaving the mission and just over-leveling, so it's possible. Not really reasonable or in keeping with presumable developer intent, but possible.
No, as in the very first time you actually see it.
Regicide
Kill an alien ruler the first time you encounter it
I honestly think the only way you can do this legitimately is to play on easy, don't do the mission where you encounter the Viper King until you have max everything, then go into the fight and stun the King with the freeze grenade so you get 3 free actions. Then shoot it as much as possible in those three actions.
In the third action it will summon the portal. Then in your fourth action you must savescum your plasma bolter to stun it. Then you must kill it before that stun runs out if not it will teleport away.
I'm assuming on easy it has less health because it has 60 normally. And two armour. That's a metric shittonne of health to kill in that short amount of time.
You'd have to be insane to try to do it to the other two who have way more health than the King. Or some seriously mod upgraded weapons / cheats.